attempt primitive ​

Signature ​

function attempt<T>(parser: Parser<T>): Parser<T>

Description ​

attempt combinator applies parser without consuming any input. It doesn't care if parser succeeds or fails, it won't consume any input.

Usage ​

The example is the same as in the docs for lookahead combinators.

const Parser = sequence(
  takeLeft(string('hello'), whitespace()),
  lookahead(string('let')),
  string('lettuce')
)

run(Parser).with('hello lettuce')

{
  isOk: true,
  span: [ 0, 13 ],
  pos: 13,
  value: [ 'hello', 'let', 'lettuce' ]
}

Notice how differs the output for the last failing case: attempt doesn't consume any input, i.e. it doesn't advance pos.

run(Parser).with('hello let')

{
  isOk: false,
  span: [ 6, 9 ],
  pos: 9, 
  expected: 'lettuce'
}

run(Parser).with('hello something')

{
  isOk: false,
  span: [ 6, 9 ],
  pos: 6, 
  expected: 'let'
}